electron devices 3
TRANSCRIPT
EC8252: Electronic Devices Department of ECE
1
2020-2021 Jeppiaar Institute of Technology
EC8252 ELECTRON DEVICES L T P C
3 0 0 3
UNIT I SEMICONDUCTOR DIODE 9
PN junction diode, Current equations, Energy Band diagram, Diffusion and drift current densities,
forward and reverse bias characteristics, Transition and Diffusion Capacitances, Switching
Characteristics, Breakdown in PN Junction Diodes.
UNIT II BIPOLAR JUNCTION TRANSISTORS 9
NPN -PNP -Operations-Early effect - Current equations – Input and Output characteristics of CE, CB,
CC - Hybrid -π model - h-parameter model, Ebers Moll Model- Gummel Poon-model, Multi Emitter
Transistor.
UNIT III FIELD EFFECT TRANSISTORS 9
JFETs – Drain and Transfer characteristics, - Current equations - Pinch off voltage and its
significance- MOSFET- Characteristics- Threshold voltage -Channel length modulation, D-MOSFET,
E-MOSFET- Characteristics – Comparison of MOSFET with JFET.
UNIT IV SPECIAL SEMICONDUCTOR DEVICES 9
Metal-Semiconductor Junction- MESFET, FINFET, PINFET, CNTFET, DUAL GATE MOSFET,
Schottky barrier diode-Zener diode-Varactor diode –Tunnel diode- Gallium Arsenide device, LASER
diode, LDR.
UNIT V POWER DEVICES AND DISPLAY DEVICES 9
UJT, SCR, Diac, Triac, Power BJT- Power MOSFET- DMOS-VMOS. LED, LCD, Photo transistor,
Opto Coupler, Solar cell, CCD.
TOTAL: 45 PERIODS
OUTCOMES:
At the end of the course the students will be able to:
Explain the V-I characteristic of diode, UJT and SCR
Describe the equivalence circuits of transistors
Operate the basic electronic devices such as PN junction diode, Bipolar and Field effect
Transistors, Power control devices, LED, LCD and other Opto-electronic devices
TEXT BOOKS:
1. Donald A Neaman, ―Semiconductor Physics and Devices‖, Fourth Edition, Tata Mc GrawHill Inc.
2012.
EC8252: Electronic Devices Department of ECE
2
2020-2021 Jeppiaar Institute of Technology
2. Salivahanan. S, Suresh Kumar. N, Vallavaraj.A, ―Electronic Devices and circuits‖, Third Edition,
Tata McGraw- Hill, 2008.
REFERENCES:
1. Robert Boylestad and Louis Nashelsky, ―Electron Devices and Circuit Theory‖ Pearson Prentice
Hall, 10th edition, July 2008.
2. R.S.Sedha, ― A Text Book of Applied Electronics‖ S.Chand Publications, 2006.
3. Yang, ―Fundamentals of Semiconductor devices‖, McGraw Hill International Edition, 1978.
EC8252: Electronic Devices Department of ECE
3
2020-2021 Jeppiaar Institute of Technology
THE ATOM
All matter is composed of atoms; all atoms consist of electrons, protons, and neutrons except normal
hydrogen, which does not have a neutron. Each element in the periodic table has a unique atomic
structure, and all atoms within a given element have the same number of protons. At first, the atom
was thought to be a tiny indivisible sphere. Later it was shown that the atom was not a single particle
but was made up of a small dense nucleus around which electrons orbit at great distances from the
nucleus, similar to the way planets orbit the sun. Neil’s Bohr proposed that the electrons in an atom
circle the nucleus in different obits, similar to the way planets orbit the sun in our solar system. The
Bohr model is often referred to as the planetary model. Another view of the atom called the quantum
model is considered a more accurate representation, but it is difficult to visualize.
The Bohr Model
An atom is the smallest particle of an element that retains the characteristics of that element.
According to the classical Bohr model, atoms have a planetary type of structure that consists of a
central nucleus surrounded by orbiting electrons, as illustrated. The nucleus consists of positively
charged particles called protons and uncharged particles called neutrons. The basic particles of
negative charge are called electrons.
Fig 1.1: The Bohr model of an atom showing electrons in orbits around the nucleus, which consists
of protons and neutrons. The “tails” on the electrons indicate motion.
EC8252: Electronic Devices Department of ECE
4
2020-2021 Jeppiaar Institute of Technology
Fig 1.2: Two simple atoms, hydrogen and helium.
Atomic Number
The atomic number equals the number of protons in the nucleus, which is the same as the number of
electrons in an electrically balanced (neutral) atom. For example, hydrogen has an atomic number of 1
and helium has an atomic number of 2.
Electrons and Shells
Energy Levels Electrons orbit the nucleus of an atom at certain distances from the nucleus. Electrons
near the nucleus have less energy than those in more distant orbits. Only discrete (separate and
distinct) values of electron energies exist within atomic structures. Therefore, electrons must orbit
only at discrete distances from the nucleus.
Each discrete distance (orbit) from the nucleus corresponds to a certain energy level. In an atom, the
orbits are grouped into energy levels known as shells. A given atom has a fixed number of shells.
Each shell has a fixed maximum number of electrons. The shells (energy levels) are designated 1, 2,
3, and so on, with 1 being closest to the nucleus.
EC8252: Electronic Devices Department of ECE
5
2020-2021 Jeppiaar Institute of Technology
Fig: Illustration of the Bohr model of the silicon atom.
The Maximum Number of Electrons in Each Shell: The maximum number of electrons (Ne) that can
exist in each shell of an atom is a fact of nature and can be calculated by the formula
n - Number of the shell
The maximum number of electrons that can exist in
Innermost shell (Shell 1) –
Shell 2 –
Shell 3 –
Valence Electron
Electrons that are in orbits farther from the nucleus have higher energy and are less tightly
bound to the atom than those closer to the nucleus. This is because the force of attraction between the
positively charged nucleus and the negatively charged electron decreases with increasing distance
from the nucleus. Electrons with the highest energy exist in the outermost shell of an atom and are
relatively loosely bound to the atom. This outermost shell is known as the valence shell and electrons
in this shell are called valence electron.
These valence electrons contribute to chemical reactions and bonding within the structure of a
material and determine its electrical properties. When a valence electron gains sufficient energy from
an external source, it can break free from its atom. This is the basis for conduction in materials
Ionization
When an atom absorbs energy from a heat source or from light, for example, the energies of the
electrons are raised. The valence electrons possess more energy and are more loosely bound to the
atom than inner electrons, so they can easily jump to higher energy shells when external energy is
absorbed by the atom.
If a valence electron acquires a sufficient amount of energy, called ionization energy, it can
actually escape from the outer shell and the atom’s influence. The departure of a valence electron
leaves a previously neutral atom with an excess of positive charge (more protons than electrons). The
process of losing a valence electron is known as ionization, and the resulting positively charged atom
is called a positive ion. For example, the chemical symbol for hydrogen is H. When a neutral
hydrogen atom loses its valence electron and becomes a positive ion, it is designated H_. The escaped
valence electron is called a free electron.
EC8252: Electronic Devices Department of ECE
6
2020-2021 Jeppiaar Institute of Technology
The reverse process can occur in certain atoms when a free electron collides with the atom and is
captured, releasing energy. The atom that has acquired the extra electron is called a negative ion. The
ionization process is not restricted to single atoms. In many chemical reactions, a group of atoms that
are bonded together can lose or acquire one or more electrons.
Drawback of Bohr Model:
It is not a complete model
The Quantum Model
Considered to be more accurate.
It is a statistical model and very difficult to understand or visualize.
Like the Bohr model, the quantum model has a nucleus of protons and neutrons surrounded
by electrons.
Two important theories under lie the quantum model: the wave-particle duality and the uncertainty
principle.
1. Wave-particle duality
2. Uncertainly principle
Orbitals:
In the quantum model, each shell or energy level consists of up to four subshells which are designated
s, p, d, and f.
Orbital s - 2 electrons
Orbital p - 6 electrons
Orbital d - 10 electrons
Orbital f - 14 electrons
Atomic Energy Level Diagram
EC8252: Electronic Devices Department of ECE
7
2020-2021 Jeppiaar Institute of Technology
MATERIALS USED IN SEMICONDUCTOR
In terms of their electrical properties, materials can be classified into three groups: Conductors,
semiconductors, and insulators. When atoms combine to form a solid, crystalline material, they
arrange themselves in a symmetrical pattern. The atoms within the crystal structure are held together
by covalent bonds, which are created by the interaction of the valence electrons of the atoms. Silicon
is a crystalline material.
INSULATOR CONDUCTORS SEMICONDUCTORS
A material that does not
conduct electrical current under
normal conditions.
A material that easily conducts
electrical current. Most metals
are good conductors.
A material that is between
conductors and insulators in its
ability to conduct electrical
current
Most good insulators are
compounds rather than single-
element materials and have very
high resistivities. Examples:
rubber, plastics, glass, mica,
and quartz.
The best conductors are single-
element materials, such as
copper (Cu), silver (Ag), gold
(Au), and aluminium (Al),
which are characterized by
atoms with only one valence
electron very loosely bound to
the atom.
A semiconductor in its pure
(intrinsic) state is neither a good
conductor nor a good insulator.
Single-element semiconductors
are antimony (Sb), arsenic (As),
astatine (At), boron (B),
polonium (Po), tellurium (Te),
Silicon (Si), and germanium
(Ge).
Valence electrons are tightly
bound to the atoms; therefore,
These loosely bound valence
electrons become free electrons.
The single-element semi-
conductors are characterized by
EC8252: Electronic Devices Department of ECE
8
2020-2021 Jeppiaar Institute of Technology
there are very few free electrons
in an insulator
Therefore, in a conductive
material the free electrons are
valence electrons.
atoms with four valence
electrons. Silicon is the most
commonly used semiconductor.
Band Gap
When an electron acquires enough additional energy, it can leave the valence shell, become a free
electron, and exist in what is known as the conduction band.
Fig 1. : Energy Band Gap in (a) Insulators (b) Semiconductors & (c) Conductors
Semiconductor
The current density due to motion of electrons
The current density due to motion of holes
The difference in energy between the valence band and the conduction band is called an energy gap or band
gap. This is the amount of energy that a valence electron must have in order to jump from the valence band to
the conduction band.
EC8252: Electronic Devices Department of ECE
9
2020-2021 Jeppiaar Institute of Technology
Total current density J in a Semiconductor
Where,
Pure Semiconductor (Intrinsic Semiconductor)
Number of free electrons = Number of Holes
Effective mass
Classification of Semiconductor
Intrinsic Semiconductor
A Semiconductor which is in extremely pure form
Extrinsic Semiconductor
EC8252: Electronic Devices Department of ECE
10
2020-2021 Jeppiaar Institute of Technology
N – type Semiconductor
To increase the number of conduction-band electrons in intrinsic silicon, pentavalent
impurity atoms are added. These are atoms with five valence electrons such as arsenic (As),
phosphorus (P), bismuth (Bi), and antimony (Sb).
Each pentavalent atom (antimony, in this case) forms covalent bonds with four adjacent
silicon atoms. Four of the antimony atom’s valence electrons are used to form the covalent bonds with
silicon atoms, leaving one extra electron. This extra electron becomes a conduction electron because it
is not involved in bonding. Because the pentavalent atom gives up an electron, it is often called a
donor atom. The number of conduction electrons can be carefully controlled by the number of
impurity atoms added to the silicon. A conduction electron created by this doping process does not
leave a hole in the valence band because it is in excess of the number required to fill the valence band.
Fig: Pentavalent impurity atom in a silicon crystal structure.
Majority and Minority Carriers Since most of the current carriers are electrons, silicon (or
germanium) doped with pentavalent atoms is an n-type semiconductor (the n stands for the negative
charge on an electron). The electrons are called the majority carriers in n-type material. Although
the majority of current carriers in n-type material are electrons, there are also a few holes that are
created when electron-hole pairs are thermally generated. These holes are not produced by the
addition of the pentavalent impurity atoms. Holes in an n-type material are called minority carriers.
P – Type Semiconductor
To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added.
These are atoms with three valence electrons such as boron (B), indium (In), and gallium (Ga). As
illustrated in Figure 1–18, each trivalent atom (boron, in this case) forms covalent bonds with four
adjacent silicon atoms. All three of the boron atom’s valence electrons are used in the covalent bonds;
and, since four electrons are required, a hole results when each trivalent atom is added. Because the
EC8252: Electronic Devices Department of ECE
11
2020-2021 Jeppiaar Institute of Technology
trivalent atom can take an electron, it is often referred to as an acceptor atom. The number of holes
can be carefully controlled by the number of trivalent impurity atoms added to the silicon. A hole
created by this doping process is not accompanied by a conduction (free) electron.
Fig: Trivalent impurity atom in a silicon crystal structure. A boron (B) impurity atom is shown in
the center.
Majority and Minority Carriers Since most of the current carriers are holes, silicon (or germanium)
doped with trivalent atoms is called a p-type semiconductor. The holes are the majority carriers in p-
type material. Although the majority of current carriers in p-type material are holes, there are also a
few conduction-band electrons that are created when electron-hole pairs are thermally generated.
These conduction-band electrons are not produced by the addition of the trivalent impurity atoms.
Conduction-band electrons in p-type material are the minority carriers.
Conductivity of Semiconductor
EC8252: Electronic Devices Department of ECE
12
2020-2021 Jeppiaar Institute of Technology
Resistivity (ρ) of a semiconductor is the reciprocal of conductivity, i.e.,
Energy Distribution of Electrons
Carrier Concentration In Intrinsic Semiconductor
To calculate the conductivity of a semiconductor, the concentration of free electrons and the
concentration of free holes must be known
dn - Number of conduction electrons per unit cubic meter whose energies lie between E & E + dE.
N(E) - Density of states
In a semiconductor the lowest energy in the conduction band is and hence
The Fermi Dirac probability function f(E) is given by
The concentration of electrons in the conduction band is,
For E >= , E - >> kT
Mass – Action Law:
Under thermal equilibrium for any semiconductor, the product of number of holes and the number of
electrons is constant and is independent of the amount of donor and acceptor impurity doping.
EC8252: Electronic Devices Department of ECE
13
2020-2021 Jeppiaar Institute of Technology
Drift Current
When an electric field is applied across the semiconductor material, the charge carriers attain a certain
drift velocity
The holes move towards the negative terminal and electrons move towards the positive terminal. This
combined effect of movement of charge carriers constitutes a current known as the drift current.
“Drift current is defined as the flow of electric current due to the motion of the charge
carriers under the influence of an external electric field”
Drift Current Density
A conducting wire of length l cm contains N electrons. If an electron travels a distance of l cm
in the conductor in time T sec, the total number of electrons passing through any cross section of wire
per second is
The total current flowing through the wire with area of cross – section A is given by
The drift current density J is defined as the current per unit area of the conducting medium.
EC8252: Electronic Devices Department of ECE
14
2020-2021 Jeppiaar Institute of Technology
The drift current density due to the charge carriers such as free electrons and holes are the current
passing through a square centimetre perpendicular to the direction of flow. The equation for the drift
current density, Jn due to free electrons is given by
drift current density, Jp due to holes is given by
Diffusion Current
It is possible for an electric current to flow in a semiconductor even in the absence of the
applied voltage provided a concentration gradient exists in the material. A concentration gradient
exists if the number of either electrons or holes is greater in one region of a semiconductor as
compared to the rest of the region. In a semiconductor material, the charge carriers have the tendency
to move from the region of higher concentration to that of lower concentration of the same type of
charge carriers. Thus, the movement of charge carriers takes place resulting in a current called
diffusion current.
Diffusion Current depends on
Material of the Semiconductor
Type of Charge Carriers
Concentration Gradient
EC8252: Electronic Devices Department of ECE
15
2020-2021 Jeppiaar Institute of Technology
Diffusion current density due to holes, is given by,
Hole density p(x) decreases with increasing x.
Diffusion current density due to free electrons, is given by,
Total current density for an P – type Semiconductor
Total current density for an N – type Semiconductor
Continuity Equation
Theory of PN Junction Diode
PN junction Diode in Equilibrium with no Applied Voltage
In a piece of semiconductor material, one half is doped by P – type impurity and the other half is
doped by N – type impurity, a PN junction is formed. The plane dividing the two halves or zones is
called PN junction.
N – type material – high concentration of free electrons
P – type material – high concentration of holes
At the junction, there is a tendency for the free electrons to diffuse over to the P – side and holes to
the N – side. This process is called Diffusion. As the free electrons move across the junction from N
– type to P – type, the donor ions become positively charged. Hence positive charge is built on the N
– side of the junction. Therefore, a net negative charge is established on the P – side of the junction.
This net negative charge on the P – side prevents further diffusion of electrons into the P – side.
Similarly, the net positive charge on the N – side repels the hole crossing from P – side to N – side.
Thus, a barrier is set – up near the junction which prevents further movement of charge carriers, i.e.,
electrons and holes. As a consequence a induced electric field across the depletion layer, an
EC8252: Electronic Devices Department of ECE
16
2020-2021 Jeppiaar Institute of Technology
electrostatic potential difference is established between P – and N – regions, which is called the
potential barrier, diffusion potential or contact potential, Vo. The magnitude of contact potential Vo
varies with doping levels and temperature.
The electrostatic field across the junction caused by the positively charged N – type region tends to
drive the holes away from the junction and negatively charged P – region tends to drive the electrons
away from the junction. The majority holes diffusing out of the P – region leave behind negatively
charged acceptor atoms bound to the lattice, thus exposing negative space charge in a previously
neutral region. Similarly, electrons diffusing from the N – region expose positively ionised donor
atoms, and a double space charge layer builds up at the junction. It is noticed that the space – charge
layers are of opposite sign to the majority carriers diffusing into them, which tends to reduce the
diffusion rate.
Calculation of Depletion Width
Consider the width of the Depletion region contains space charge since, donors on the N –
side and acceptors on the P – side have lost their accompanying electrons and holes. Hence, an
electric filed is established which, in turn, causes a difference in potential energy, q .
The potential variation in the space charge region can be calculated by using poisson’s equation,
which is given by
Applying the above equation to the P – side of the junction, we get
Integrating twice we get,
Where C and D are the constants of integration.
V = 0 at x = 0 and hence D = 0. When x<X1 on the P – side, the potential is constant, so that
. Hence
EC8252: Electronic Devices Department of ECE
17
2020-2021 Jeppiaar Institute of Technology
As V = V1 at x = X1, we have
If we apply the same procedure to the N – side, we get
Therefore, the total built – in potential or the contact potential is Vo, where
We know that the positive charge on the N – side must be equal in magnitude to the negative charge
on the P – side for the neutral specimen.
Substituting this relationship in the above equation
The total depletion width, and hence,
, and then substituting
for from the above equations, we find
Here, in an alloy junction,
EC8252: Electronic Devices Department of ECE
18
2020-2021 Jeppiaar Institute of Technology
The depletion region width W is proportional to
Under Forward Bias Condition
Under Forward Bias Condition
When positive terminal of the battery is connected to the P – type and the negative terminal to the N –
type of the PN junction diode, the bias applied is known as forward bias.
Operation
The applied potential with external battery acts in opposition to the internal potential barrier and
disturbs the equilibrium. As soon as the equilibrium is disturbed by the application of an external
voltage, the fermi level is no longer continuous across the junction. The applied positive potentials
repel the holes in the P – type region so that the holes moves towards the junction and the applied
negative potentials repels the electrons in the N – type region and the electrons move towards the
junction. Eventually, when the applied potential is more than the internal barrier potential, the
depletion region and internal potential barrier disappears.
V – I characteristics of a diode under Forward Bias
Forward Voltage ( ) is increased
Forward current ( ) is almost zero (region OA)
The potential barrier prevents the holes from P – region and electrons from N – region to flow across
the depletion region in the opposite direction.
- The potential barrier at the junction completely disappears and hence, the holes cross the
junction from P – type to N – type and the electrons cross the junction in the opposite direction,
resulting in relatively large current flow in external circuit.
For Germanium (Ge) – 0.3 V, Silicon – 0.7 V.
Under Reverse Bias Condition
When negative terminal of the battery is connected to the P – type and the positive terminal to the N –
type of the PN junction diode, the bias applied is known as reverse bias.
Operation
Under applied reverse bias holes which form the majority carriers of the P – side move towards the
negative terminal of the battery and electrons which form the majority carrier of the N – side are
attracted towards the positive terminal of the battery. Hence, the width of the depletion region which
is depleted of mobile charge carriers increases.
EC8252: Electronic Devices Department of ECE
19
2020-2021 Jeppiaar Institute of Technology
V – I characteristics of a diode under Reverse Bias
Energy Band Structure of Open – Circuited PN junction
Fig: Energy – band Structure
Contact difference of Potential
A contact difference of potential exists across an open circuited PN junction.
W.K.T
From the above equation, we get
We know that for N – type material
We know that for P – type material
EC8252: Electronic Devices Department of ECE
20
2020-2021 Jeppiaar Institute of Technology
As then the contact difference of potential or barrier voltage is given by
Quantitative theory of PN Diode Currents
Let us now drive the expression for the total current as a function of applied voltage assuming
that the width of the depletion region is zero. When a forward bias is applied to a diode, holes are
injected from the P – side into the N – side. Due to this, the concentration of holes in the N – side (pn)
is increased from its thermal equilibrium value (pno) and injected hole concentration [Pn(x)] decreases
exponentially with respect to distance (x).
Injected hole concentration at x=0 is
These several components of hole concentration in the N – side of a forward biased diode, in which
the density pn(x) decreases exponentially with distance (x).
This is the Boltzmann’s relation of kinetic gas theory. This equation is valid as long as the hole
current is small compared with diffusion or drift current. This condition is called low level injection.
EC8252: Electronic Devices Department of ECE
21
2020-2021 Jeppiaar Institute of Technology
Open Circuit Condition (i.e., V=0)
Under forward bias condition let V be the applied voltage, then the effective barrier voltage
The hole concentration throughout the P – side is constant and equal to the thermal equilibrium value
. The hole concentration varies exponentially with distance into the N – side.
At x=0,
Equation (4) can be changed into
Comparing Eqn (5) & (6)
This boundary condition is called the law of the junction. Substituting this into eqn (3), we get
The diffusion hole current in the N – side is
Forward Currents
The hole current crossing the junction into the N – side with x = 0 is
The electron current crossing the junction into the P – side with x = 0 is
EC8252: Electronic Devices Department of ECE
22
2020-2021 Jeppiaar Institute of Technology
The total Diode Current
Where
Reverse Saturation Currents
We know that
Applying these relationships in the above equation of reverse
saturation current, Io we get
Diode Current Equation
Transition or Space Charge (Depletion Region) Capacitance (
Under reverse bias condition, the majority carriers move away from the junction, thereby uncovering
more immobile charges. Hence, the width of the space – charge layer at the junction increases with
reverse voltage. This increase in uncovered charge with applied voltage may be considered a
capacitive effect. The parallel layers of oppositely charged immobile ions on the two sides of the
junction form the capacitance, CT which is expressed as
EC8252: Electronic Devices Department of ECE
23
2020-2021 Jeppiaar Institute of Technology
dQ is the increase in charge caused by a change in voltage dV.
A change in voltage dV in a time dt will result in a current given by
Step – graded function
The total charge density of a P – type material with area of the junction A is given by
Differentiating the above equation w.r.t. V, we get
Types of Configuration
a) CB Configuration
b) CE Configuration
c) CC Configuration